Fail if cask source already exists.

Library/Homebrew/cask/lib/hbc/artifact/moved.rb

@@ -45,8 +45,7 @@ module Hbc
           if force
             ohai(warning_target_exists { |s| s << "overwriting." })
           else
-            ohai(warning_target_exists { |s| s << "not moving." })
-            return false
+            raise CaskError, warning_target_exists << "."
           end
         end
         unless source.exist?

Library/Homebrew/cask/lib/hbc/artifact/symlinked.rb

@@ -39,7 +39,7 @@ module Hbc
 
       def preflight_checks(source, target)
         if target.exist? && !self.class.islink?(target)
-          ohai "It seems there is already #{self.class.artifact_english_article} #{self.class.artifact_english_name} at '#{target}'; not linking."
+          opoo "It seems there is already #{self.class.artifact_english_article} #{self.class.artifact_english_name} at '#{target}'; not linking."
           return false
         end
         unless source.exist?

Library/Homebrew/cask/lib/hbc/cli/install.rb

@@ -35,6 +35,8 @@ module Hbc
           rescue CaskNoShasumError => e
             opoo e.message
             count += 1
+          rescue CaskError => e
+            onoe e.message
           end
         end
         count.zero? ? nil : count == cask_tokens.length